“Metal nanoparticles” are typically fine particles of a sole metal of a diameter of 5 to 15 nm or so. Compared with the conventional particles of diameters of the order of several μm or more, they have an overwhelmingly larger specific surface area and thereby an extremely large activity, so are promising for diverse applications such as a material for storage of hydrogen or other gases. In particular, transition metal nanoparticles are promising for diverse areas due to their high chemical activity.
However, for example, as proposed in Japanese Patent Publication (A) No. 2004-168641, various ways are known for producing nanoparticles of metal oxides, but metals, in particular transition metals, alone have an extremely high chemical and physical activity, so end up easily forming compounds or aggregates. It was extremely difficult to stably produce nanoparticles.